FIG. 32 is a plan schematic illustrative of a flying type magnetic head and a positioning mechanism therefor used in a conventional magnetic or hard disk drive (HDD).
As shown in FIG. 32, the magnetic head is built up of a slider 2 having an electromagnetic transducer element 1, and a suspension 3 for supporting the slider 2. For the purpose of simplification, interconnecting lines to make electric connections between the electromagnetic transducer element 1 and a signal processing circuit in the hard disk drive for the transmission of read/write signals are not illustrated.
A VCM (voice coil motor) 5 is used in an actuator for magnetic head positioning. The VCM 5 is built up of a coil 51, a permanent magnet 52, a bearing 53 and an arm 54. The arm 54 is provided with the coil 51 at one end and with the suspension 3 of the magnetic head at the other end. It is noted that, although not illustrated, another permanent magnet is provided on the coil 51.
The electromagnetic transducer element 1 comprises a magnetic pole and coil for converting electric signals to magnetic signals, and vice versa, and a magnetoresistance effect element for transforming magnetic signals into a voltage change, etc., each being fabricated by thin film techniques, assembly techniques, etc. The slider 2 is formed of non-magnetic ceramics such as Al.sub.2 O.sub.3 --TiC or CaTiO.sub.3 or a magnetic material such as ferrite, and has a generally cuboidal shape. The surface (air bearing surface) of the slider 2 opposite to a disk medium 6 is processed into a shape suitable for generating pressure to fly the slider 2 on the disk medium 6 at a small spacing. The suspension 3 is formed by bending, punching or otherwise processing a resilient stainless sheet.
Then, an account is given of the recording, and reproducing operations of the magnetic head. The disk medium 6 rotates at high speed, e.g., several thousand rpm, and so an amount of air enters between the disk medium 6 and the slider 2 to apply flying force on the slider 2. On the other hand, the slider 2 is pressed by the suspension 3 toward the disk medium 6 under a given load, so that the slider 2 is filed on the disk medium 6 at a small spacing on the basis of a flying force vs. pressure relation.
The magnetic head is connected to the VCM 5, and so is movable in the radial direction of the disk medium 6 by its swing motion around the bearing 53, allowing positioning control of the electromagnetic transducer element 1, i.e., seek control for moving the element 1 onto any read/write track, and read/write track-following control.
For positioning control of the electromagnetic transducer element 1, the electromagnetic transducer element 1 first detects a track position signal recorded in the disk medium 6. Then, the signal is operated in a head positioning control circuit 7 so that a given current is passed through the coil 51 in the VCM 5 via an amplifier 8 to control force generated between the coil 51 and the permanent magnet 52. Upon positioned on the target track, the electromagnetic transducer element 1 writes and reads magnetic signals in and from the disk medium 6.
As mentioned above, it has so far been general to use a voice coil motor as magnetic head positioning means.
A problem with a magnetic disk drive is a track misregistration that is an offset between the magnetic head and the recording tracks caused by vibrations of the disk medium surface in synchronism or a synchronism with the rotation of the disk medium, eccentricity of the disk medium, thermal expansion and extraneous vibrations of the magnetic disk drive including the magnetic head and disk medium, etc. This track misregistration leads to problem, for instance, erasion upon recording of signal information stored in adjacent tracks due to overwriting, a drop upon reproduction of the level of signals outputted from the track concerned, and a quality drop of output signals due to the entrance of signals from adjacent tracks in the form of crosstalk noises.
For read/write purposes, it is thus required to allow the position of the electromagnetic transducer element in the magnetic head to follow a given recording track on the disk medium precisely and rapidly.
A grave problem attendant to the conventional magnetic disk drive is, however, that there is a limit in positioning precision, especially recording track-following precision due to the swing motion of the whole of a massive structure comprising the magnetic head, arm and coil, the movement of the slider via the resilient suspension, and the fact that the bearing providing the center of the swing motion has in itself friction resistance, eccentricity, and so on.
On the other hand, a magnetic disk drive is increasingly required to have ever-higher recording density, and so have ever-higher track density and ever-narrower track width and, with this, it is required to make magnetic head positioning precision ever-higher. So far, a positioning control bandwidth is up to about 500 Hz and positioning precision is about 0.3 .mu.m. As the recording track width becomes as narrow as about 1 .mu.m, however, it is required that the control bandwidth be extended to a few kHz and the positioning precision be on the order of about 0.1 .mu.m or less. For these reasons, the problems associated with a conventional magnetic head positioning mechanism become even more troublesome.
The flying height of the magnetic head is generally within the range of about 50 nm to about 100 nm. As the read/write track pitch becomes as narrow as about 1 .mu.m while such a flying height is maintained, however, several problems arise, for instance, erasion upon recording of signal information stored in adjacent tracks due to a recording magnetic field leakage, and an S/N degradation upon reproduction due to a drop of signal level absolute output. To maintain stable read/write characteristics, it is therefore required to reduce the flying height of the magnetic head with respect to the disk medium to 50 nm or lower. As the flying height decreases, however, it is required to more severely control the flying characteristics (fluctuations in the flying height) of the magnetic head upon seek control or read/write track-following control, and the smoothness of the disk medium.
In recent years a magnetic head called a pseudo-contact type magnetic head having a very small flying height or a contact type head that is always in contact with a disk medium, too, is under development. For these magnetic heads taking aim at achieving much higher recording densities, it is very difficult to conduct seek control or read/write track-following control.
For one approach to improving magnetic head positioning precision, it has been proposed to provide a magnetic head mounting arm or supporting spring (suspension) with a micro-displacement actuator such as a piezoelectric element (see JP-A's 5-28670 and 5-47216). With this approach some improvement may be introduced in the magnetic head positioning precision. However, this approach, too, is unavoidably affected by vibrations of the suspension because the slider is driven via the suspension having resiliency as in the case of the aforesaid VCM positioning mechanism. Thus there is a limit in positioning precision improvements.
For slider displacement, it has also been put forward to use an electrostatic force microactuator ("Magnetic Recording Head Positioning at Very High Track Densities Using a Microactuator-Based, Two-Stage Servo System", IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, Vol. 42, No. 3, pp. 222-233, June 1995) or an electromagnetic force microactuator ("Silicon Microstructures and Microactuators for Compact Computer Disk Drives", IEEE CONTROL SYSTEMS, pp. 52-57, December 1994). However, it is found that with the electrostatic force actuator it is difficult to generate driving force large enough to displace the slider, and with the electromagnetic force actuator there is concern that the magnetic signal on a disk medium is affected by flux leakage. A problem common to both actuators is that they are sensitive to extraneous disturbances because the slider must be held in place by means of a support member that has small resiliency and can be deformed by electrostatic force, and electromagnetic force generated from the actuators. Such a problem is also found in JP-A 8-180623 disclosing that a slider is driven by an electrostatic actuator.
Further, JP-A 6-259905 discloses an actuator for a read/write system, in which a slider comprising a read/write function part (an electromagnetic transducer element) is coupled to a suspension driven by a voice coil motor via a driving member such as a piezoelectric element. In the first embodiment (see FIG. 1) described therein, a thin type micro-motion mechanism part (a driving member) is built up of a set of opposite thin sheets and a set of piezoelectric elements. Both ends of each sheet are diagonally provided with flexures for connecting thereto either one of both ends of each piezoelectric element, and the terminals of the piezoelectric elements are connected to such flexures. With this type, of micro-motion mechanism part, both the piezoelectric elements are deformed in such a manner that they elongate or contract, thereby providing micro-displacement of the read/write function part in a direction substantially perpendicular to the elongation, and contraction direction of the piezoelectric elements. In the second embodiment (see FIG. 2) described in the publication, a thin type micro-motion mechanism part is again built up of a set of opposite thin sheets and a set of piezoelectric elements. However, this second embodiment is different from the first embodiment in that a flexure for connecting one end of each piezoelectric element is provided at one end of each thin sheet. At the thin type micro-motion mechanism part, each piezoelectric element is bendable within a plane parallel with the flying surface of the slider and in the same direction, thereby providing micro-displacement of the read/write function part, as in the first embodiment. In the third embodiment (see FIG. 3) described in the publication, a micro-motion mechanism part is built up of a set of bendable piezoelectric elements alone. In other words, each piezoelectric element is connected at one end with the slider and at the other end with the suspension.
JP-A 6-259905 refers to a micro-motion actuator with a piezoelectric element embedded in a slider itself as a prior art example, and notes that such a slider (a micro-motion actuator) has an adverse influence on its flying characteristics because its flying surface is twisted, distorted or otherwise deformed. In connection with the advantages of the invention, on the other hand, the publication notes that since no stress is applied at all on the slider at the aforesaid thin type micro-motion mechanism part and so no deformation of the slider occurs, it is possible to construct an actuator having no adverse influence at all on the flying behavior of the magnetic head. However, the first and second embodiments of the thin type micro-motion mechanism part described therein are each of an assembly structure wherein a set of thin sheets are connected together with a set of piezoelectric elements, assuming that there is no characteristic difference between both piezoelectric elements. Nonetheless, the magnetic head is susceptible to displacement in the flying direction (fluctuations in the flying height) due to assembly errors, etc. Assembly steps required for the assembly structure incur some considerable cost, and the assembly structure renders it difficult to increase its rigidity, resulting in a mechanical robustness drop problem associated with bonded parts. In addition, the assembly structure becomes poor in displacement characteristics due to the need of drawing interconnecting lines out of the electrodes of the piezoelectric elements, and costs much as well thanks to its complexity. The third embodiment, too, poses similar problems, for instance, variations in displacement characteristics due to assembly errors.
It is noted that the actuator shown in FIG. 6 of JP-A 6-259905 as an prior art example, and the actuator set forth in JP-A 5-28670 has a structure wherein displacement generating means such as piezoelectric elements are fitted into a suspension. For such a structure, it is difficult to place the size of the displacement generating means and the size of the spaces in which they are fitted under tolerance control. For piezoelectric elements having a very small displacement amount, it is undesirable that a gap exists between them because there is loss or a variation of the amount of displacement transmitted, or otherwise no displacement is transmitted to the actuator whatsoever. To eliminate such a gap, high-precision assembly techniques are need, but not only do they involve technical difficulties but incur some considerable expense as well.
JP-A 63-291271 discloses a magnetic head positioning arrangement including a micro-motion mechanism comprising a piezoelectric element or an electrostrictive element between a slider and a load bar. The micro-motion mechanism set forth in the example of the publication comprises a thin piezoelectric element sheet provided with a C-shaped cutout. By elongating or contracting a part of the piezoelectric element sheet, the slider can be linearly displaced in the same direction as the direction of elongation and contraction. This micro-motion mechanism is of an integrated structure rather than an assembly structure, wherein the amount of displacement of the slider depends on the amount of linear displacement of the piezoelectric element sheet. To increase the amount of displacement of the slider, therefore, there is no choice but to increase the size of the micro-motion mechanism.
In the arrangement illustrated in FIGS. 1 to 4 of JP-A 5-47126, a sheet form of piezoelectric element itself is bonded to a head supporting spring, and so the head supporting spring provides a load that hampers the displacement of the piezoelectric element. For this reason, there is a further decrease in the small amount of displacement of the piezoelectric element. In addition, no sufficient robustness is achievable because some load is applied on the piezoelectric element, resulting in a reduction of its life, and a premature degradation of its performance. When an adhesive layer usually comprising an organic resin, etc. is used for the bonding of the piezoelectric element, the following problems arise. Transmission loss occurs because the displacement of the piezoelectric element gives rise to deformation of the adhesive layer; bonding reliability drops because some load is applied on the adhesive layer upon the displacement of the piezoelectric element; and displacement transmitted from the piezoelectric element to the electromagnetic transducer element through the adhesive layer is susceptible to a secular change because the adhesive force (fixing force), and rigidity of the adhesive layer changes depending on the aforesaid load, and ambient conditions (high temperature, high humidity, and so on).
Illustrated in FIG. 6 of JP-A 6-309822 is a read/write head comprising driving means of the structure in which piezoelectric elements, each in a sheet form, are mounted on a pair of metal thin sheets forming parallel leaf springs of a gimbal member formed at a tip of a suspension. According to such driving means, upon the elongation and contraction of the piezoelectric elements there is a difference between their length and the length of the metal thin sheets, which in turn allows both the elements and the sheets to be so deflected that an electromagnetic transducer element can be displaced. In this arrangement, too, there is again a reliability drop because the piezoelectric elements deflect the metal thin sheets through an adhesive layer.
JP-A 60-47271 discloses a flying head having a driving element between a slider and a head body, said driving element providing micro-motion of the head body in a direction of intersecting the parallel information tracks. As this driving element a piezoelectric element is mentioned. Typically, the publication refers to a flying head making use of a thickness change of a multilayer piezoelectric element to provide micro-motion of the head body, and a flying head making use of a bimorph type piezoelectric element. The head body set forth in the publication is an ordinary bulk type magnetic head, rather than a thin film head type of electromagnetic transducer element. The multilayer piezoelectric element described in the publication utilizes displacement in the same direction as an electric field direction, i.e., piezoelectric longitudinal effect, which gives rise to a thickness-wise linear displacement to displace the head body linearly. A problem with this flying head is, therefore, that the size of the piezoelectric element must be increased to increase the amount of displacement of the head body. Another problem with this flying head is that a part of the flying surface is susceptible to deformation, and displacement as well. This leads to further problems that the flying characteristics are unstable, and the read/write characteristics are unstable as well due to fluctuations in the spacing between the head body and the medium.
While explanation has been made with reference to a magnetic head out of read/write heads, it is understood that the problems mentioned in connection with magnetic head positioning are all true of a recording/reproducing head for optical disk systems. A conventional optical disk system makes use of an optical pickup comprising an optical module including at least a lens. This optical pickup is so designed that the lens can be mechanically controlled so as to be focused on the recording surface of the optical disk. In recent years, near field recording has been proposed to achieve ever-higher optical disk recording densities. In this regard, see "NIKKEI ELECTRONICS", Jun. 16, 1997 (No. 691), page 99. This near field recording makes use of a flying head which uses a slider like a slider used with a flying type magnetic head. Built in this slider is an optical module comprising a hemispherical lens called a solid immersion lens or SIL, a magnetic field modulation recording coil, and a prefocusing lens. Another flying head for near field recording is disclosed in U.S. Pat. No. 5,497,359. An optical disk used in combination with such a flying head has very high recording track densities. Accordingly, when positioning is effected with respect to recording tracks, the same problems as mentioned with reference to the flying type magnetic head arise.